This invention relates to solder assembly of components.
Soldering of components is often a critical operation in the fabrication of circuits and devices since failure to form an effective solder joint can result in costly yield and reliability problems. These problems are especially prevalent when an area array of solder joints is needed. For example, in certain high performance connectors, a flexible printed circuit may be used to electrically connect a contact element to a rigid printed circuit board. For high density applications, the flexible circuit might preferably terminate in an area array which must be electrically coupled to corresponding terminations on the printed circuit board. One technique for attachment involves the use of conductive pins inserted in holes formed in the corresponding terminations in the flexible circuit and circuit board. (See, for example, U.S. Pat. No. 4,806,107, issued to Arnold et al, which is incorporated by reference herein.) It would be more desirable to solder the flexible circuit directly to the circuit board since that would provide for improved electrical performance of the circuits and eliminate the need for extra piece parts.
Solder assembly of a flexible circuit to a rigid printed circuit board is typically done by solder plating the ends of the conductive lines on the flexible circuit and on the circuit board, and then using a hot ram to reflow the plating and form a linear arrangement of solder joints between corresponding lines. A major problem is that solder joint formation is often not reliable, and many mis-formed joints would not be detectable until the final testing of the device since such joints would be hidden under the component.
It is, therefore, an object of the invention to provide a method of soldering which results in high yields and readily inspectable and repairable solder joints.
Another concern in soldering of components is to provide just the right amount of solder to the pad arrays for forming an effective solder joint. Inadequate amount of solder can lead to problems in forming reliable solder joints with high yield, and excessive solder can result in formation of solder bridges between adjacent conducting elements and hence the need for repair. Plating often does not deposit an adequate solder volume dependably. Solder dipping is a possible approach to providing an enhanced solder coating on components, but the lack of control of solder volume leads to lack of control over the assembly process which produces assembly yield problems. Another approach, which is often used for surface mounted components, is to deposit the solder in the form of a paste through apertures in a stencil. Control of the deposited solder in such cases can become a problem depending on the ratio of the aperture width and thickness of the stencil. This is because solder paste can cling to the stencil rather than deposit on the circuit board, and tacky deposits can build up on the walls of the apertures and interfere with solder deposition.
It is, therefore, a further object of the invention to provide a method and apparatus for depositing a well-controlled amount of solder in a desired location.